bootstrap uses downloaded rustc despite lockfile changes

[RalfJung]

I have run cargo update -p rustc_apfloat to link a newer apfloat into Miri. download-rustc is set to "if-unchanged". However, it seems bootstrap doesn't consider this to "change" rustc: ./x test miri still uses the downloaded rustc, leading to test failures since I didn't get the newer apfloat I was hoping for.

Does bootstrap somehow have the lockfile on the list of files it can ignore for download-rustc? That would not make sense, would it?

Cc @onur-ozkan

[onur-ozkan]

Cargo.lock was added to the allowlist to avoid invalidating download-rustc when making dependency changes in the library tree. Would doing this only on the library profile solve your problem?

[RalfJung]

I think it is a pretty severe bug for "if-unchanged" to ever be wrong, no matter the profile. Correctness is more important than performance. If I have to debug "if-unchanged" issues any time something odd happens, it doesn't actually save time. It is pure luck that it only took me ~10min to realize that it was using the wrong version of the crate that I just updated, this could have easily been an hour-long debugging session!

Please do not ever add anything to the allowlist that could possibly influence the compiler build. This is crucial. If we don't have reliable foundations for building rustc, how are we supposed to build a reliable compiler?

Also, the library anyway has its own lockfile these days.

[onur-ozkan]

I thought that would be pretty rare case to update dependencies of compiler without touching on compiler tree but yeah, that was a mistake; we shouldn't add that to the allow list.

Also, the library anyway has its own lockfile these days.

My bad, it was src/librustdoc. Maybe they should have separate Cargo.lock files too. I will raise this on Zulip later today. Feel free to revert #142232.

[RalfJung]

It is indeed rare for me, not sure about others. But we don't accept rare soundness bugs or rare miscompilations, so equally we should not accept rare build system malfunctions. I'm not sure when I will have time to post a revert, debugging this has used up a lot of my Rust time for today. I'd appreciate if you could do this.

[Kobzol]

Posted a revert here: #142344